A transoesophageal echocardiogram, or TOE (TEE in the United States) utilizes a special probe containing an ultrasound transducer in its tip, which is passed into the patient's oesophagus. This procedure is considered to be superior to the more traditional non-invasive transthoracic echocardiogram since it is able to produce clearer images, especially of structures that are otherwise difficult to view transthoracicly. Since the heart rests directly upon the oesophagus leaving only millimeters in distance that the ultrasound beam has to travel, there is little attenuation of the ultrasound signal, thereby generating a stronger return signal. This results in an enhanced image and improved ultrasound quality.
It is generally recognized that several pulmonary structures can be evaluated and imaged with TOE, including the aorta, pulmonary artery, valves of the heart, both atria, atrial septum, left atrial appendage, as well as the coronary arteries. Recently, however, the principles underlying TOE have been expanded and improved upon. In particular, oesophageal Doppler monitoring has been developed and used to combat the condition known as hypovolaemia, which is a pronounced reduction in circulating blood volume. Hypovolaemia may result from the combined effects of pre-operative fasting, the anaesthetic agent and blood loss during the surgical procedure. The complications that hypovolaemia causes arise because the reduced circulating blood volume is unable to carry sufficient oxygen to the major organs and tissues. Patients undergoing surgery are constantly at risk from this serious and potentially life-threatening condition.
Oesophageal Doppler monitoring is premised on the Doppler effect, which in the broadest sense is the change in frequency and wavelength of a wave as perceived by an observer moving relative to the source of the waves. In the context of the cardiovascular system, the Doppler effect can be used to measure the speed and direction of blood flow leaving the heart. This information may then be used to detect any reduction in circulating blood volume early and in real-time. This allows the anesthetist to intervene quickly and safely to correct the situation, using a combination of specialized fluids and drugs, before the hypovolaemia becomes more serious.
The insertion of prior art Doppler probes for TOE and oesophageal Doppler monitoring procedures is unfortunately very uncomfortable to the awake patient. As shown in FIG. 1A, the Doppler probe 120 is attached to the end of a shaft 115, which is being used to manually advance the probe tip 120 into and through the patient's nasal cavity 105, past the pharynx, into the oesophagus 110 itself, and ultimately to a level of between the 5th and 6th thoracic ribs (referred to as the “T5 to T6 level”). It should be noted that the probe tip 120 of FIG. 1A is depicted in an intermediate or transition state during insertion, but prior to reaching its focused position at the T5 to T6 level.
Proper placement and orientation of the probe tip 120 is critical to the monitoring process. As such, in order to be able to properly manipulate the position and orientation of the probe tip 120 once inserted, the prior art shaft 115 is made to be resiliently bendable. This resiliency or rigidity is imparted using a tightly wound spring which traverses the length of the shaft 115.
In addition to the rigidity and resulting abrasiveness of the probe tip 120 itself, the resilient nature of the prior art shaft 115 tends to exert a constant and sizable force against the wall of the oesophagus in the general vicinity of area 125. For example, the typical prior art shaft (e.g., shaft 115) would exert a force on the order of 120 gram-force (gf) against the oesophagus wall. FIG. 1B shows that, in addition to the discomfort experienced by patients along the walls of the oesophagus (i.e., area 125), the top of the nasal cavity 130 will also tend to be a source of discomfort since the inwardly-directed pressure is concentrated around this area, and essentially used to force the shaft downward through the pharynx and into the oesophageal area.
Exacerbating the discomfort associated with Doppler probe insertions is that, unlike traditional TOE procedures, oesophageal Doppler monitoring at times cannot be performed while the patient is sedated. For example, it has been found that oesophageal Doppler monitoring can be beneficial during awake-patient surgery and during post-operative recovery periods. Not only does the awake state dramatically increase the physical discomfort associated with the oesophageal Doppler monitoring procedure, the patient's gag reflex may also complicate successful completion of the procedure itself.
Thus, there is a need in the art for an improved oesophageal Doppler monitoring probe and system which reduces the discomfort associated with probe insertion.